Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Ullmannite is a nickel antimony sulfide mineral with formula: NiSbS. Considerable substitution occurs with cobalt and iron in the nickel site along with bismuth and arsenic in the antimony site. A solid solution series exists with the high cobalt willyamite. It is steel-gray to tin white in color with a metallic luster. It has a Mohs hardness of 5 to 5.5 and a specific gravity of 6.65. It crystallizes in the isometric crystal class and typically exhibits cubic, octahedral, or pyritohedral forms although euhedral crystals are rare. Crystal twinning is common.